1. Field of the Invention
The present invention relates generally to transmission lines, and particularly to transitions between different kinds of transmission lines.
2. Technical Background
Electronic, electro-optic and other devices for high-speed operation at ultra-high microwave frequencies ( greater than 10 GHz) are difficult to design because interconnections have unintentional capacitance and inductances, causing undesirable side effects. Simple low frequency interconnects cause attenuation and other parasitic distortions of the microwave signal and therefore the interconnects have to be designed and treated as transmission lines for frequencies higher than the radio frequency (RF) range, including the ultra-high microwave frequencies. Transmission lines, such as microstrip and coplanar waveguides (CPW) are generally not combined on the same substrate. However, to form larger subsystems, such as electro-optic modulators or other high-speed devices, there is a need to be able to connect dissimilar transmission lines, such as a wider CPW signal conductor to a narrower microstrip conductor, with a manufacturable broadband transition that has a minimum and smooth return loss of at least 15 dB across a range of at least DC to 50 GHz.
One example of a larger subsystem is the top surface planar packaging electrode connection to the electrodes of an electro-optic (EO) chip. It is known that high-speed operation of electro-optic (EO) waveguide modulators requires RF transmission lines for the modulator driving electrodes to achieve velocity matching of the electrical and optical signals and to overcome the capacitance limitations of a lumped element drive electrode. Preferably, these transmission lines should have characteristic impedances (Z0) equal to or near 50 Ohms for matching to the drive electronics. Broadband operation is also a requirement of these modulators. According to well-known transmission line theory, the characteristic impedance is dependent on the dielectric between the lines. In general, the optimum geometries for an EO polymer modulator where the dielectric is a polymer, the drive electrode and the lines by which the drive signal is routed into the device package are dissimilar. Therefore, well-designed transitions from one type of RF transmission line to another are usually necessary for efficient, broadband operation of the modulator. Many types of transitions are known. However, none of the known transitions have tied together all of the essential elements for a broadband (DC to 50 GHz), uniplanar CPW to MS transition having a smooth low-return loss, in the context of the unique requirements for driving a high-speed electro-optic (EO) polymer modulator.
Therefore, there is a need for a high frequency, broadband uniplanar transition wherein the transition lies on the same plane/surface as the interconnecting center conductors of two dissimilar transmission line segments for the examplary purpose of driving an EO polymer modulator.
One aspect of the present invention is a broadband interconnection device used for interconnection between a first transmission line and a second transmission line, having a substrate with the first transmission line defined at a first side on a first surface, the first transmission line including a signal conductor and at least one ground conductor, a signal conductor of the second transmission line defined on an opposite side of the first surface, and a ground plane of the second transmission line on an opposed surface, the signal conductor of the first transmission line being electrically connected to the signal conductor of the second transmission line on the first surface. On the opposed surface, the ground plane of the second transmission line, has at least one protrusion aligned with the signal conductor of the first transmission line.
In another aspect, the present invention includes a second ground shape of a second ground of a second transmission line on a second plane is geometrically configured to interact with a first ground of a first transmission line on a first plane for maintaining a uniform desired characteristic impedance for broadband microwave signal propagation between the first and second transmission lines.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.